Drill bit with cutting elements having functionally engineered wear surface

ABSTRACT

Drill bits and cutting elements having a functionally-engineered surface comprise a cermet material selected from the group consisting of refractory metal carbides, nitrides, borides, carbonitrides and mixtures thereof. A functionally-engineered material is disposed over a surface portion one of the cutting elements to form a wear resistant surface thereon having a hardness that is different than that of the underlying cutting element. The wear resistant surface is provided by forming a conformable material mixture by combining one or more powders selected from the group consisting of cermets, carbides, borides, nitrides, carbonitrides, refractory metals, diamond particles, cubic boron nitride particles, Co, Fe, Ni, and combinations thereof, with an applying agent. The applied material mixture is pressurized under conditions of elevated temperature to consolidate and sinter the material mixture, thereby forming the wear resistant surface having desired properties of hardness and/or fracture toughness.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of U.S. patent applicationSer. No. 10/454,374, filed on Jun. 3, 2003, which application is herebyincorporated in its entirety and that was a continuation of U.S. patentapplication Ser. No. 09/846,944, filed on May 1, 2001, now issued asU.S. Pat. No. 6,571,889, which application claimed the priority of U.S.Provisional Application No. 60/200,851, filed on May 1, 2000.

FIELD OF THE INVENTION

This invention relates to drill bits used for subterranean drilling and,more particularly, to rotary cone bits with cutting elements havingfunctionally-engineered composite surfaces, and methods for forming thesame, having improved mechanical properties of wear resistance andtoughness when compared to conventional rotary cone bits.

BACKGROUND OF THE INVENTION

Cemented tungsten carbide, such as WC—Co is well known for itsmechanical properties of hardness, toughness and wear resistance, makingit a popular material of choice for use in such industrial applicationsas mining and drilling where its mechanical properties are highlydesired. Because of its desired properties, cemented tungsten carbidehas been the dominant material used on rotary cone rock bit surfacesexposed to wear, e.g., on cutting inserts used with rotary cone rockbits. The mechanical properties associated with cemented tungstencarbide and other cermets, especially the unique combination ofhardness, toughness and wear resistance, make these materials moredesirable than either metals or ceramics alone.

It is known in the art that for cemented tungsten carbide, fracturetoughness is inversely proportional to hardness, and wear resistance isproportional to hardness. Accordingly, when using cemented tungstencarbide as a wear surface one must balance the demand for high wearresistance with the desire to have an acceptable degree of fracturetoughness. A cemented tungsten carbide material having a high degree ofwear resistance may not provide a sufficient degree of fractureresistance for drilling applications, resulting in a wear surface thatis brittle and thus susceptible to gross brittle fracture. A cementedtungsten carbide material having a high degree of fracture resistance,while not being brittle and having acceptable impact resistance, may nothave a suitable degree of wear resistance for drilling applications.

A known approach for addressing this issue of competing desiredproperties has been to use a cemented tungsten carbide substrate, andplace a cemented tungsten carbide material over the substrate to providea relatively more wear resistant surface thereon. In this approach, thecemented tungsten carbide placed on the substrate is speciallyformulated to provide a greater degree of wear resistance than that ofthe underlying substrate, and the substrate is formulated to provide agreater degree of fracture toughness than the surface layer. Thecemented tungsten carbide used as the wear surface is bonded to thesubstrate and consolidated by the process of liquid phase sintering.

A known limitation with this approach, however, is that the interfacebetween the substrate and the surface layer must be flat or planar.Thus, this approach is not useful for addressing the need to provide awear surface formed from cemented tungsten carbide, having both adesired degree of wear resistance and fracture toughness, on a substratehaving an irregular or nonplanar interface surface, e.g., an interfacesurface having a variable or constant radius of curvature.

A further known limitation with this approach is the reliance uponliquid phase sintering to bond the cemented tungsten carbide substrateand surface layer together. During the process of liquid phase sinteringit is known that the ductile metal component, e.g., cobalt metal,liquefies and migrates across the boundary or interface between thesubstrate and surface layer. This migration is not desired because itreduces the intended differential between the two material compositionsacross the interface, causing the interface to become homogeneous andthe related differential material properties to be minimized oreliminated. For example, during liquid phase sintering the cobalt metalconstituent in the substrate can migrate into the surface layer, whereless of the cobalt metal constituent is desired to provide the desireddegree of wear resistance. In this instance, such migration causes anundesired reduction in the wear resistance provided by the surfacelayer. Thus, this phenomenon of liquid phase migration is known to limitthe ability to control surface layer properties by use of a materialdifferential approach.

Cemented tungsten carbide constructions known in the art are typicallyformed into the shape of a green part in sheet form that is sintered toan underlying substrate during the above-described liquid phaseconsolidation process. The above-described process of forming the greenpart and the finally-sintered product both limits the types ofconstructions that can be used to form the final product, e.g.,constructions comprising complex microstructures or multiple layers maybe outside the scope of practical manufacturing capabilities, and limitsthe types of products that can include the complex construction, e.g.,products having an irregular shape or a nonplanar substrate surface(such as those developed by residual stress analysis), may also beoutside of the scope of practical manufacturing capabilities. In manyrotary cone rock bit applications, it is desired that a portion of thebit or cutting element having a nonplanar surface comprising a layer ofcemented tungsten carbide disposed thereon for purposes of improvingwear resistance and fracture toughness at that location.

It is, therefore, desired that functionally-engineered compositesurfaces, for use with rotary cone rock bits, be prepared according toprinciples of this invention in a manner that does not adversely impactthe physical properties of either the substrate or the surface material,e.g., in a manner that avoids ductile phase metal migration, whencompared to wear resistant surfaces applied by liquid phase sinteringmethod. It is desired that such functionally-engineered compositesurfaces be formed in a manner that permits use on substrates havingirregular or nonplanar interface geometries. It is further desired thatfunctionally-engineered composite surfaces of this invention provide animproved degree of wear resistance and fracture toughness when comparedto conventional cemented tungsten carbide surfaces formed using liquidphase sintering methods.

SUMMARY OF THE INVENTION

Functionally-engineered composite wear surfaces, prepared according toprinciples of this invention, are provided on cutting elements used withrotary cone rock bits, and can be specially engineered to provide adesired degree of wear resistance and/or fracture toughness necessary tomeet particular drilling applications. Cutting elements comprisingfunctionally-engineered composite wear surfaces of this invention areformed in a manner that both avoids unwanted material migration, betweenthe wear surface and substrate, and that permits application onnonplanar, e.g., curved, interface surfaces, thereby enabling placementof wear surfaces where not before practical.

As mentioned above, functionally-engineered composites of this inventionare used with rotary cone bits that comprise a bit body having at leastone leg extending therefrom, and a cone that is rotatably disposed onthe leg. The cone typically comprises a plurality of cutting elementsthat project outwardly therefrom. The cutting elements may comprise acermet material selected from the group consisting of refractory metalcarbides, nitrides, borides, carbonitrides and mixtures thereof.

A functionally-engineered material is disposed over a surface portion ofat least one of the cutting elements to form a wear resistant surfacethereon. The wear resistant surface has a hardness that is differentthan that of the underlying cutting element. The wear resistant surfaceis formed by forming a conformable material mixture by combining one ormore powders selected from the group consisting of cermets, carbides,borides, nitrides, carbonitrides, refractory metals, Co, Fe, Ni, andcombinations thereof, with an applying agent.

As used herein, the term “conformable” is used to describe the nature ofthe mixture as being in a physical state that readily conforms to aninterface surface of the substrate, e.g., being in the form of asemi-plastic material or a liquid slurry. The conformable materialmixture is applied to an interface surface of the cutting element toprovide a green state material layer thereon. Depending on theparticular application, the material mixture can be applied in the formof a coating onto the interface surface or, prior to application, can bepreformed into a part shaped to fit over the interface surface, which islater applied over the interface surface.

The applied material mixture is pressurized under conditions of elevatedtemperature to consolidate and sinter the material mixture, therebyforming the wear resistant surface. The material mixture is consolidatedand sintered in a manner that avoids unwanted material migration betweenthe applied material mixture and substrate, thereby providing afully-densified wear surface having the desired properties of hardnessand/or fracture toughness.

DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention willbecome appreciated as the same becomes better understood with referenceto the specification, claims and drawings wherein:

FIG. 1 is a perspective side view of a rotary cone rock bit comprising afunctionally-engineered surface layer provided according to principalsof the invention;

FIG. 2 is a fragmentary, longitudinal cross-section of the rotary conerock bit of FIG. 1; and

FIG. 3 is a schematic perspective side view of a cutting element usedwith the rock bit of FIG. 1 comprising a functionally-engineered surfacelayer provided according to principles of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Rotary cone bits, prepared according to the practice of this invention,comprise functionally-engineered surfaces formed from compositeconstructions that are applied in a manner that minimizes both unwantedmaterial migration from the construction (thereby retaining desiredcomposite construction material properties) and surface irregularities(thereby ensuring desired surface wear properties and service life).Further, such functionally-engineered surfaces are applied in a mannerthat permits their use on substrates having nonplanar or irregularsurface geometries.

Composite constructions used for forming functionally-engineeredsurfaces on rotary cone bits comprise a material microstructure havingone or more material phases. The terms “functionally engineered” as usedherein refers to the fact that the composite construction materialmicrostructure has been specifically designed and manufactured toprovide one or more particular physical properties, e.g., wearresistance and/or fracture toughness, suited for a particular finalproduct application.

Functionally-engineered surfaces formed according to principles of thisinvention are intended to be used on substrates that are exposed toextreme conditions of wear and impact, such as subterranean drillingbits for purposes of extending the service life of such equipment. FIG.1 illustrates a rotary cone rock bit comprising afunctionally-engineered surface of this invention. The rock bitcomprises a body 10 having three cutter cones 11 mounted on its lowerend. A threaded pin 12 is at the upper end of the body for assembly ofthe rock bit onto a drill string for drilling oil wells or the like. Aplurality of inserts 13 is pressed into holes in the surfaces of thecutter cones for bearing on the rock formation being drilled. Nozzles 15in the bit body introduce drilling fluid into the space around thecutter cones for cooling and carrying away formation chips drilled bythe bit.

FIG. 2 is a fragmentary, longitudinal cross-section of a rock bit,extending radially from the rotational axis 14 of the rock bit throughone of the three legs on which the cutter cones 11 are mounted. Each legincludes a journal pin extending downwardly and radially, inwardly onthe rock bit body. The journal pin includes a cylindrical bearingsurface having a hard metal insert 17 on a lower portion of the journalpin. The hard metal insert can be formed from a variety of hard metals,as better described below, and is welded in place in a groove on thejournal leg, and has a substantially greater hardness that the steelforming the journal pin and rock bit body.

An open groove 18 is provided on the upper portion of the journal pin.Such a groove may, for example, extend around 60 percent or so of thecircumference of the journal pin, and the hard metal insert 17 canextend around the remaining 40 percent or so. The journal pin also has acylindrical nose 19 at its lower end.

Each cutter cone 11 is in the form of a hollow, generally-conical steelbody having inserts 13 that provide the drilling action by engaging asubterranean rock formation as the rock bit is rotated. As described inbetter detail below, such cutting inserts can be formed from acarbide-containing material and have a functionally-engineered surfaceprepared according to principles of this invention to provide enhancedwear resistance and improved fracture toughness.

The cavity in the cone contains a cylindrical bearing surface includingan aluminum bronze insert 21 deposited in a groove in the steel of thecone or as a floating insert in a groove in the cone. The aluminumbronze insert 21 in the cone engages the hard metal insert 17 on the legand provides the main bearing surface for the cone on the bit body. Anose button 22 is between the end of the cavity in the cone and the nose19 and carries the principal thrust loads of the cone on the journalpin. A bushing 23 surrounds the nose and provides additional bearingsurface between the cone and journal pin. Other types of bits,particularly for higher rotational speed applications, have rollerbearings instead of the journal bearings illustrated herein. It is to beunderstood that O-ring seals constructed according to principles of thisinvention may be used with rock bits comprising either roller bearingsor conventional journal bearings.

A plurality of bearing balls 24 are fitted into complementary ball racesin the cone and on the journal pin. These balls are inserted through aball passage 26, which extends through the journal pin between thebearing races and the exterior of the rock bit. A cone is first fittedon the journal pin, and then the bearing balls 24 are inserted throughthe ball passage. The balls carry any thrust loads tending to remove thecone from the journal pin and thereby retain the cone on the journalpin. The balls are retained in the races by a ball retainer 27 insertedthrough the ball passage 26 after the balls are in place. A plug 28 isthen welded into the end of the ball passage to keep the ball retainerin place.

The bearing surfaces between the journal pin and the cone are lubricatedby a grease. Preferably, the interior of the rock bit is evacuated, andgrease is introduced through a fill passage (not shown). The grease thusfills the regions adjacent the bearing surfaces plus various passagesand a grease reservoir, and air is essentially excluded from theinterior of the rock bit. The grease reservoir comprises a cavity 29 inthe rock bit body, which is connected to the ball passage 26 by alubricant passage 31. Grease also fills the portion of the ball passageadjacent the ball retainer, the open groove 18 on the upper side of thejournal pin, and a diagonally extending passage 32 therebetween. Greaseis retained in the bearing structure by a resilient seal in the form ofan annular seal 44 between the cone and journal pin.

A pressure compensation subassembly is included in the grease reservoir29. The subassembly comprises a metal cup 34 with an opening 36 at itsinner end. A flexible rubber bellows 37 extends into the cup from itsouter end. The bellows is held into place by a cap 38 with a ventpassage 39. The pressure compensation subassembly is held in the greasereservoir by a snap ring.

Rotary cone bits such as those illustrated in FIGS. 1 and 2 comprise afunctionally-engineered surface formed from a composite construction andprepared according to principles of this invention. Thefunctionally-engineered surface can be positioned along any surface ofthe bit calling for an improved degree of wear resistance. Typically,functionally-engineered surfaces of this invention are applied toportions of the rock bit cone and/or cutting elements, e.g., cuttinginserts or steel teeth in a milled tooth bit.

FIG. 3 illustrates a cutting element removed from the rock bit in theform of an insert 50. The insert 50 comprises a generally cylindricalbody 52 that is formed from a material that is relatively harder thanthe cone, e.g., a steel or carbide-containing material such as cementedtungsten carbide. The insert 50 includes a functionally-engineered wearsurface 54, prepared according to principles of this invention, disposedalong an axial end of the insert opposite the end that is attached tothe cone.

Functionally-engineered wear surfaces of this invention are formed fromcomposite constructions that can be in the form of: (1) a coating havinga single layer, made up of one or more materials, that is disposed ontoa substrate surface; (2) a coating of two or more layers, each made upof one or more material, that are disposed onto a substrate surface; or(3) a part made up of one or more materials that itself forms the finalproduct used to perform the product application. Referring to theembodiment illustrated in FIG. 3, the wear surface 54 can comprisesingle or multiple layers of materials disposed onto the substrate.Alternatively, the substrate itself, i.e., the insert body, can beformed entirely from the desired composite construction material, inwhich case inherently providing the functionally-engineered wear surfacealong its axial end.

Suitable materials that can be used as substrates forfunctionally-engineered wear surface of this invention include thoseselected from the group including steel materials, steel alloymaterials, cermet materials, and mixtures thereof. The term “cermet”, asused herein, is understood to refer to those materials having both aceramic and a metallic constituent. Such cermet materials comprise amaterial microstructure that is characterized by a plurality of hardphase grains that are bonded together by a metallic cementing agent.Example cermet materials that can be used as substrates include thosecomprising a hard grain constituent formed from carbides or borides ofrefractory metals such as W, Ti, Mo, Nb, V, Si, Hf, Ta, Cr, and ametallic cementing agent. Example hard grain materials include WC, TiC,TaC, TiB₂, or Cr₂C₃. The metallic cementing agent may be selected fromthe group of ductile materials including one or a combination of Co, Ni,Fe, W, Mo, Ti, Ta, V and Nb, which may be alloyed with each other orwith C, B, Cr or Mn. Preferred cermets useful for forming the substrateinclude cemented tungsten carbide with cobalt as the binder phase(WC—Co), WC—Ni, WC—Fe, WC—(Co, Ni, Fe) and alloys thereof.

Another acceptable type of cermet that can be used to form substratesused with functionally-engineered wear surfaces of this invention is onehaving a double cemented cermet material microstructure. The term“double cemented” as used herein refers to the fact that the materialmicrostructure for such construction comprises a plurality of firstphases made up of a cermet, distributed within a substantiallycontinuous second phase made from a relatively more ductile material.Because the each first phase comprises hard grains bonded together orcemented by a metallic cementing agent, and the first phases arethemselves disposed within a second metallic cementing agent, theoverall material microstructure is referred to as being double cemented.

Example double cemented cermet constructions useful for formingsubstrates include those disclosed in U.S. Pat. No. 5,880,382, which isincorporated herein by reference, and which have a materialmicrostructure comprising a plurality of first phases (each formed fromthe same types of cermet materials discussed above) that are distributedwithin a substantially continuous matrix second phase that is formedfrom a relatively more ductile material (such as those materialdiscussed above useful for forming the cermet metallic cementing agent).

Additionally, substrates used with functionally-engineered wear surfacesof this invention can be formed from a diamond carbide compositematerial, such as that disclosed in U.S. patent application Ser. No.09/521,717 filed on Mar. 9, 2000, which is incorporated herein byreference.

In a preferred embodiment, the substrate is formed from a doublecemented cermet construction is one comprising first phases formed fromcemented tungsten carbide (WC—Co), and a second phase formed fromcobalt. More preferably, the substrate comprises a first phasecomprising cemented tungsten carbide having the following compositionalrange: carbide component in the range of from about 75 to 97 percent byweight, and metallic cementing agent or binder in the range of fromabout 3 to 25 percent by weight.

Substrates used with functionally-engineered wear surfaces of thisinvention are in a preexisting rigid state at the time that the wearsurface is applied thereto. For example, substrates in the form of arotary cone or a cutting element are either forged and machined fromsteel bars, i.e., in the form of wrought or casting stock, or aresintered from metal powders, i.e., in the form of a fully- orpartially-densified substrate.

Functionally-engineered wear surfaces of this invention are preparedfrom composite constructions comprising the following materials: cermetsof carbides, nitrides, carbonitrides, and borides; steel;polycrystalline diamond particles, cubic boron nitride particles; andmixture thereof. Cermet materials useful for forming thefunctionally-engineered wear surface include the group of cermetmaterials described above for forming the substrate.

An important feature of the functionally-engineered wear surface of thisinvention is that it displays a degree of hardness that is differentthan that of the underlying substrate. In an example embodiment, whereproperties of high wear resistance is desired, it is desired that thecomposite construction selected to form the functionally-engineered wearsurface provide a hardness (as measured using the Rockwell A scale) thatis at least 0.5 greater than that of the underlying substrate. Thisminimum level of increased hardness is desired because hardnessdifferences less than this would not show significant propertydifferentials, in terms of wear versus toughness relationships.

In another example embodiment, where properties of high fracturetoughness is desired, it is desired that the composite constructionselected to form the form the functionally-engineered wear surfaceprovide a hardness (as measured using the Rockwell A scale) that is atleast 0.5 less than that of the underlying substrate. Accordingly, it isto be understood that embodiments prepared according to principals ofthis invention can comprise functionally-engineered surfaces having adegree of hardness that can be greater or less than that of thesubstrate, depending on particular product application.

A key feature of this invention is the ability to provide thefunctionally-engineered wear surface onto a substrate interface surfacehaving an irregular or nonplanar geometry. For example,functionally-engineered wear surfaces of this invention can be appliedto a concave or convex substrate interface having a constant orirregular radius of curvature. For example, functionally-engineered wearsurfaces of this invention can be applied to interface surfaces having aradius of curvature that range from infinity (flat) to 0.7 mm, dependingon the particular application, e.g., insert size, with a preferredminimum radius of curvature being approximately 1.5 mm.

Another key feature of this invention is the ability to provide thefunctionally engineered wear surface onto a substrate without theundesired migration of materials, i.e., a ductile metal constituent,between the material interfaces. These features are accomplished byusing methods (both of application and consolidation) that are differentthan those known in the art. Such methods not only provide theabove-identified features, but also enable formation of functionallyengineered wear surfaces having complex microstructures and/or multiplelayers. These methods include polymer-assisted forming methods,thermoforming methods, and injection molding methods. Each of thesemethods is described in greater detail below as follows:

Polymer-Assisted Forming

Generally speaking, polymer-assisted forming comprises the process ofusing one or more different applying agents in the form of polymers ororganic binders to aid in forming a solution, from constituent materialpowders, which solution is then used to form a green part, e.g., forforming a coating onto an identified substrate surface. The use of anapplying agent is desired as it introduces flexibility into the processof making a green part by enabling formation of a semi-plastic solutionthat can either be spray applied or dip applied onto the substratesurface to form a desired coating. Further, the use of polymer-assistedforming enables the creation of multi-layer composite constructions thatcan comprise the same or different materials in each layer, and enablesthe formation of coatings that have accurately controlled layerthicknesses.

Polymer-assisted forming involves the process of: (1) combining adesired material powder useful for forming the functionally-engineeredwear surface, e.g., cermet powder, diamond particles, cubic boronnitride particles, carbides, nitrides, carbonitrides, borides, metals,metal alloys, and mixtures thereof, with an applying agent; (2) mixingthe material powder and applying agent together to form a semi-plasticsolution having a desired consistency; and (3) applying the solution toa desired substrate surface by dip, spray, brush, or roll technique.Alternatively, the material solution can comprise a double cementedcarbide as disclosed in U.S. Pat. No. 5,880,382, incorporated herein byreference, or a diamond carbide composite as disclosed in U.S. patentapplication Ser. No. 09/521,717 filed on Mar. 9, 2000, which isincorporated herein by reference.

Suitable polymer binders useful as applying agents for thepolymer-assisted forming process include those capable of blending withthe material powder to form a substantially homogeneous mixture andproviding flexibility to the solid material (powder) to facilitateshaping. Additionally, the chosen polymer should have a desirableburnout behavior, enabling it to be removed from the product duringprocessing without causing damage to the structure.

Example polymer binders include can include thermoplastic materials,thermoset materials, aqueous and gelation polymers, as well as inorganicbinders. Suitable thermoplastic polymers include polyolefins such aspolyethylene, polyethylene-butyl acetate (PEBA), ethylene vinyl acetate(EVA), ethylene ethyl acetate (EEA), polyethylene glycol (PEG),polysaccharides, polypropylene (PP), poly vinyl alcohol (PVA),polystyrene (PS), polymethyl methacrylate, methylethyl ketone (MEK),poly ethylene carbonate (PEC), polyalkylene carbonate (PAC),polycarbonate, poly propylene carbonate (PPC), nylons, polyvinylchlorides, polybutenes, polyesters, waxes, fatty acids (stearic acid),natural and synthetic oils (heavy mineral oil), and mixtures thereof.Suitable thermoset plastics useful as the polymer binder includepolystyrenes, nylons, phenolics, polyolefins, polyesters, andpolyurethanes. Suitable aqueous and gelation systems include thoseformed from cellulose, alginates, polyvinyl alcohol, polyethyleneglycol, polysaccharides, water, and mixtures thereof. Silicone is anexample inorganic polymer binder.

Polymer-assisted forming can be used to provide a surface coating onsubstrates formed from the variety of materials disclosed above havingeither a planar or nonplanar interface surface. After the green part isformed, i.e., the substrate surface is coated with the slurry material,the coated surface is consolidated by high-temperature/high pressureprocess described below, to provide the desired fully-sinteredfunctionally-engineered wear surface.

Polymer-assisted forming according to principles of this invention isbetter understood with reference to the following examples:

Dip Coating—WC—Co Dip Coated onto a WC—Co Substrate

A coating solution is prepared by combining in the range of from 10 to70 percent by volume material powder selected from the group describedabove, with the remaining volume being the polymer binder. Using lessthan about 10 percent by volume material powder would provide a materialmixture having too little solids to provide a surface layer havingdesired is properties of wear or fracture toughness for most drillingapplications. Using greater than about 70 percent by volume materialpowder would provide a material mixture that was too thick to facilitateproper application, and that could produce unwanted surfaceirregularities in the finally-formed surface layer.

In an example embodiment, the functionally-engineered wear surface isformed from cemented tungsten carbide. Thus, in the range of from 10 to70 percent by volume WC—Co powder (comprising approximately 5 to 27percent by weight Co and preferably 10 percent by weight Co) is mixedwith a remaining volume percent of polymeric binder solution. Ifdesired, WC powder and Co powder can be used instead of WC—Co powder. Ina preferred embodiment, in the range of from 50 to 75 percent by volumeof the WC—Co powder is used. In this example the polymer binder solutioncomprises approximately 20 percent by weight poly-propylcarbonate inmethyl ethyl ketone (MEK) solution. The embodiment can use bindersolutions containing from 5 to 50 weight percent polymer in solution.Moreover, solvents other than MEK may be utilized.

The polymer binder solution was combined with the material powderelement and the ingredients were mixed together using a ball mill orother mechanical mixing means for approximately 8 hours. If desired,additional solvents or other types of processing additives, such aslubricants or the like, can be used to aid in the processability of thesolution to control solution viscosity and/or to control desired coatingthickness. The resulting solution has a semi-fluid consistency.

In this example embodiment, the substrate is formed from cementedtungsten carbide. As discussed above, the underlying substrate has ahardness that is at least 0.5 (as measured by Rockwell A hardness) lessthan that of the fully-sintered functionally-engineered wear surface.The substrate is dipped into the milled solution for a period of timethat will vary depending on the make-up of the solution. In the exampleembodiment, where the material powder is WC—Co and the polymer bindercomprises MEK present in the above identified proportions, the substratesurface is dipped into the solution for a period of approximately 5seconds. The dipped surface is removed from the solution and allowed todry for a period of time, e.g., in the example embodiment, approximately1 minute. Again, drying time is understood to vary depending on theparticular solution make up.

In the example described above, the substrate is an insert used with adrill bit for subterranean drilling (as illustrated in FIG. 3), and thecoating solution is dip coated onto a working surface of the substrateto provide improved properties of wear resistance thereto. Using the dipcoating process, a desired and accurately controllable surface layercoating thickness can be achieved by single or repeated dipping process.

Additionally, a multi-layer coating construction can be achieved on thesurface of the substrate by repeating the dipping process usingdifferent dip coating solutions. This can be done, for example, tocreate a multi-layer substrate surface coating having a series oftransition layers with gradient levels or one or more material, movingfrom the coating surface to the substrate surface, to provide a gradientof physical properties through the thickness of the coating.

The above-described dip process enables placement of a single-layercoating on a substrate surface having a thickness in the range of fromabout 0.04 to 3 millimeters, and enables placement of a multi-layercoating on a substrate surface having a thickness in the range of from0.06 to 10 millimeters.

Spray Coating—WC—Co Spray Coated onto a WC—Co Substrate

A coating solution for spray coating can be prepared in the same manneras described above for dip coating, i.e., using the same proportions andtypes of material powder and polymeric binder solution. In anotherexample embodiment, a coating solution useful for spray coatingcomprises approximately 60 percent by volume WC—Co powder (comprisingapproximately 10 percent by weight Co), and approximately 40 percent byvolume IPA (using Nylon) or MEK (comprising approximately 20 percent byweight polypropylcarbonate). The material powder element and the twoingredients were mixed together using a ball mill. If desired, solventsor other types of processing additives, such as lubricants or the like,can be used to aid in the processability of the solution to controlsolution viscosity and/or to control desired coating thickness.

The resulting solution is sprayed onto a substrate surface and thejust-applied surface coating is allowed to dry as described above. Likethe dip coating process, a desired surface layer coating thickness canbe achieved by repeating the spraying process. Additionally, amulti-layer coating construction can be achieved on the surface of thesubstrate by repeating the spraying process using different spraycoating solutions, as described above for the dip coating process. Theabove-described spray coating process enables placement of asingle-layer coating on a substrate surface having a thickness in therange of from 0.04 to 3 millimeters, and enables placement of amulti-layer coating on a substrate surface having a thickness in therange of from 0.06 to 10 millimeters.

Molding Techniques

Generally speaking, molding techniques useful for formingfunctionally-engineered wear surfaces of this invention comprise processsteps that enable the manufacture of composite constructions havingsimple or complex microstructures of one or more material phases.Further, molding techniques of this invention can be used to form eitherwear surfaces for use on a substrate, or can be used to form thesubstrate itself.

The types of materials useful for forming functionally-engineered wearsurfaces using these molding techniques include the same materialsdiscussed above. Applying agents, e.g., in the form of polymer binders,can be combined with these materials to enhance processability, e.g., tofacilitate making a preformed green part by pressing and/or extrusionprocess. The composite construction formed by such molding technique canhave a microstructure of two or more material phases arranged in eitheran ordered or random manner. Examples of functionally-engineered wearsurfaces having an ordered or oriented material microstructure comprisea first region of a hard material dispersed within a second phase of arelatively more ductile material, as discussed in detail in U.S. patentapplication Ser. No. 08/903,668, which is incorporated herein byreference. Further examples of functionally-engineered compositeconstructions having an ordered or oriented microstructure are discussedin detail in U.S. patent application Ser. No. 09/499,929 filed on Feb.8, 2000, and U.S. patent application Ser. No. 09/521,717 filed on Mar.9, 2000, which are each incorporated herein by reference.

Molding techniques of this invention are better understood withreference to the following examples:

Thermoforming—Random Material Microstructure—WC—Co Cap on WC—CoSubstrate

A preformed green part is manufactured by mixing, pressing and/orextruding to form a composite construction having a randommicrostructure. The material microstructure can comprise one, or two ormore material phases. In this example embodiment, a preformed green partin the form of a cap (for placement over substrate in the form of acutting insert) is produced. The desired functionally-engineered wearsurface comprises cemented tungsten carbide, and the underlyingsubstrate (insert body) is formed from cemented tungsten carbide havinga relatively lower hardness, i.e., a higher fracture toughness.

Tungsten carbide and cobalt powders are mixed together at lowtemperature (130° C.), with an applying agent or polymer mixture. Thetotal mixture may comprises in the range of from about 30 to 90 volumepercent of solids, depending on particle characteristics and theparticular polymer system. In an example embodiment, the polymer mixturecomprises in the range of 10 to 60 percent of the total mixture volume.In a preferred embodiment, the polymer mixture comprises approximately55 percent of the total mixture volume. A preferred polymer mixture forthis example comprises approximately 90 percent by weight ethyl acetate(EEA) and 10 percent by weight white paraffin oil (otherwise known asHMO—heavy mineral oil). The mixture is cooled to room temperature andfragmented into small pieces.

The pieces are loaded into a heating mold operated at 120° C. that isused to shape the heated pieces into the form of a cap. The contents ofthe mold are pressurized, causing the heated mixture to yield and flowinto the mold, thereby conforming to the mold shape. The pressure rangesfrom 100 to 10,000 psi, and the operation last for only a few seconds.The assembly is then cooled to room temperature, preferably underpressure, and is the preformed green part is removed from the mold.

The preformed green part is positioned onto the intended substratesurface, thermally processed to drive out the polymer mixture, and isconsolidated by high temperature/high pressure process described belowto provide the desired functionally-engineered wear surface. Suchthermoforming process can be used to provide functionally-engineeredwear surfaces on substrate having layer thicknesses that are greaterthan that provided by dip coating, e.g., in the range of from 0.1 mm to25 mm.

Thermoforming—Random Material Microstructure—WC—Co w/Diamond Granules onWC—Co Substrate

A preformed green part is manufactured by mixing, pressing and/orextruding to form a composite construction having a random materialmicrostructure. The material microstructure can comprise one, or two ormore material phases. For example, a preformed green part having arandom microstructure of two different material phases can bemanufactured by combining a desired material powder (e.g., syntheticdiamond powder) with a suitable polymer binder to form a first mixture,and by combining another desired material powder (e.g., WC/Co powder)with a suitable polymer binder to form a second mixture. The firstmixture is granulized into diamond granules. The granules are combinedwith the second mixture and are pressed to form a composite constructionhaving a microstructure of diamond granules dispersed within asubstantially continuous matrix of WC/Co, as described in U.S. patentapplication Ser. No. 09/521,717 filed on Mar. 9, 2000.

The resulting preformed composite construction, comprising suchmicrostructure, is formed into a thin disk that is loaded into a press.The disk composite construction is thermoformed into a final greenproduct, either for placement onto a substrate surface or for formingthe substrate itself, by pressing under temperature conditions in therange of from 30 to 150° C., and under pressure conditions in the rangeof from 100 to 10,000 psi. For example, the thin disk can be pressed toform a green part having the shape of a cap that is placed over andsintered (by high temperature/high pressure process) to an insert usedwith a drill bit (see FIG. 3) to form a working surface thereon.

Thermoforming—Cellular/Oriented Material Microstructure

A preformed green part is manufactured by mixing, pressing and/orextrusion process to form a composite construction having an orderedcellular microstructure. The microstructure can comprise one, or two ormore material phases. For example, a preformed green part having amicrostructure of two different material phases can be manufactured bycombining a desired material powder (e.g., synthetic diamond powder)with a suitable polymer binder to form a first mixture, and by combininganother desired material powder (e.g., WC/Co powder) with a suitablepolymer binder to form a second mixture.

Both of the resulting mixtures are then pressed into suitable shapes(e.g., a rod and a shell), and are pressed together and extruded to forman initial composite (e.g., comprising a rod of diamond surrounded by ashell of WC/Co), which is taken and combined with other such initialcomposites and pressed, extruded and recombined to arrive at the finalcomposite comprising a cellular microstructure of a plurality of diamondmaterial phases surrounded by a substantially continuous WC/Co materialphase, e.g., as described in U.S. patent application Ser. No.08/903,668.

The resulting preformed composite construction, comprising such cellularmicrostructure, is formed into a thin disk that is loaded into a press.The disk composite construction is thermoformed into a final greenproduct, either for placement onto a substrate surface or for formingthe substrate itself, by pressing under temperature conditions in therange of from 30 to 150□ C, and under pressure conditions in the rangeof from 100 to 10,000 psi. For example, the final green product can bein the form of a cap that is placed onto and sintered (by hightemperature/high pressure process) to an insert used with a drill bit toform a working surface of the insert for subterranean drilling.

Thermoforming—Free Assembly Microstructure

A preformed green part is manufactured by mixing, pressing and/orextrusion process to form a composite construction having an orderedmicrostructure of multiple material phases. For example, a preformedgreen part having a random microstructure of two different materialphases can be manufactured by building one or more multilayer laminateconstructions of different materials, e.g., synthetic diamond powder,WC/Co powder, and the like, taking one or more slice of the multilayerlaminate constructions, and arranging the slices to form a compositeconstruction having a surface made up of the multiple layers.

The resulting preformed composite construction, comprising suchmicrostructure, is formed into a thin disk that is loaded into a press.The disk composite construction is thermoformed into a final greenproduct, either for placement onto a substrate surface or for formingthe substrate itself, by pressing under temperature conditions in therange of from 30 to 150□ C, and under pressure conditions in the rangeof from 100 to 10,000 psi. For example, the final green product can bein the form of a cap that is placed onto and sintered to an insert usedwith a drill bit to form a working surface of the insert forsubterranean drilling.

Powder Injection Molding

A powder injection molding mixture is prepared by combining in the rangeof from 40 to 70 percent by percent by volume material powder selectedfrom the group of materials described above for formingfunctionally-engineered wear surfaces of this invention, within therange of from 30 to 60 percent by volume polymer binders and optionallubricant/surfactant additives. In an example embodiment, 55 to 60percent by volume WC—Co powder (comprising approximately 10 percent byweight Co) is mixed with a polymer binder. A polymer binder comprising amixture of polypropylene (30 to 35 percent by weight of the binder),paraffin wax (60 to 65 percent by weight of the binder), and stearicacid (up to about 5 percent by weight of the binder) is used. Theabove-identified ingredients are mixed together in a heated mixers, suchas planetary mixer or sigma-blade mixer.

The resulting mixture is then granulated into granules having a desiredparticle size, and the granules are loaded into an injection moldingmachine operated at approximately 150° C. The granules are injected intoa shaped mold to provide a particularly configured green part, and canbe can be operated at low pressure conditions, e.g., at less than about50 MPa, or at high-pressure conditions at greater than about 50 MPa,depending on the ingredients used. For example, the granules can beloaded into a mold that is shaped to provide a green part in the form ofa cap that is configured to fit over an intended substrate surface,e.g., a rock bit cutting insert. The green part can be consolidated andsintered by high temperature/high pressure process described below toyield the desired functionally-engineered wear surface.

Each of the green parts formed according to the above-describedpolymer-assisted forming and thermoforming methods are consolidated andsintered using solid-state sintering techniques that avoid the undesiredeffects inherent with liquid phase sintering, such as material migrationbetween boundaries of different material types. Prior to sintering,however, the green part is heated to about 200 to 400° C. in vacuum orflowing reactive and/or inert gases to debind and drive off the polymercomposite. During the subsequent sintering process the debinded greenpart and substrate are both heated to an elevated temperature that isbelow the melting point of the binder phase material used to form thefunctionally-engineered wear surface, e.g., in an example embodimentbelow the melting temperature of cobalt.

Solid-state consolidation techniques useful for formingfunctionally-engineered wear surfaces of this invention include hotpressing, hot isostatic pressing (HIPing) as described in U.S. Pat. No.5,290,507 that is incorporated herein by reference, and rapidomnidirectional compaction (ROC) as described in U.S. Pat. Nos.4,945,073; 4,744,943; 4,656,002; 4,428,906; 4,341,577 and 4,124,888,which are each incorporated herein by reference.

Broadly speaking, the ROC process involves forming a mixture from thedesired precursor materials, e.g., WC hard grains and a ductile metalbinder in the event that the desired final material is a cementedtungsten carbide, along with a temporary wax binder. The mixture ispressed in a closed die to a desired shape, such as a rock bit insert ora cap that forms a working surface of a rock bit insert. The resulting“green” insert is vacuum dewaxed and presintered at a relatively lowtemperature to achieve a density appreciably below full theoreticaldensity. The presintering is only sufficient to permit handling of theinsert for subsequent processing. The green insert is wrapped in a firstcontainer and is then placed in second container made of a hightemperature high pressure self-sealing ceramic material. The secondcontainer is filled with a special glass powder and the green partdisposed within the first container is embedded in the glass powder. Theglass powder has a lower melting point than that of the green part, orof the ceramic die. The second container is placed in a furnace to raiseit to the desired consolidation temperature, that is also above themelting point of the glass. For example, for a green part formed fromWC—Co, the consolidation temperature is in the range of from 1,000° C.to 1,280° C. The heated second container with the molten glass and greenpart immersed inside is placed in a hydraulic press having a closedcylindrical die and a ram that presses into the die. Molten glass andthe green part are subjected to high pressure in the sealed ceramiccontainer. The part is isostatically pressed by the liquid glass topressure as high as 120 ksi. The temperature capability of the entireprocess can be as high as 1,800° C., depending on the particular typesof materials selected to form the green part. The high pressure isapplied for a short period of time, e.g., less than about five minutesand preferably one to two minutes, and isostatically compacts the greenpart to essentially 100 percent density.

Conventional liquid phase consolidation techniques are generally notthought to be useful for forming composite constructions of thisinvention because of the tendency for the binder material to migrate,thereby causing the material phases to become distorted or unoriented.However, liquid phase consolidation techniques may be used that operateunder conditions of reduced temperature or where the structures beingbonded differ only in grain size or hard material while matrix levelsare similar. For example, reactive liquid phase sintering relates to aprocess whereby one or more of the constituent elements is capable ofreleasing energy upon formation (i.e., enthalpy formation is high). Thisenergy is released as heat which can (if conditions are proper) producea self-propagating reaction that will consolidate the component at lowtemperature (that being the temperature needed to initiate thereaction). Thus, functionally-engineered wear surfaces of this inventioncan be sintered using this technique if one of the material phasescontains an element that, upon reaching an ignition temperature, willoperate to densify the entire composition. This technique isnonreversible, meaning that the reaction product will not go to a moltenstate at the low temperature used to initiate the reaction due to anincreased melting point of the compound in comparison to its constituentelements.

Other solid-state consolidation techniques useful for making ordercomposite constructions of the invention include those incorporating arapid heating step such as microwave sintering, plasma-activatedsintering, and other types of field-assisted sintering. Each of thesetechniques have can be effective at producing a final compositeconstruction having the retained oriented or ordered microstructure.

Examples of consolidation techniques using rapid heating methods includefield-assisted sintering and laser heating. Field-assisted sinteringused an electromotive field to generate rapid heating and improvessurface transport. Often, energy is concentrated on surface asperities.Several heating techniques for conducting field-assisted sinteringexist, including but not limited to induction heating, microwave, plasmaand electric discharge. Induction sintering uses alternating current tocreate a magnetic field with the die material to induce eddy currents.These eddy currents serve to rapidly heat a component. Similarly,microwave sintering allows for rapid heating of a component based on its(or susceptor) material properties. A susceptor is a material that willdo the heating in either induction or microwave when the compact iseither nonconductive or transparent to microwave. Besides rapid heating,microwave sintering is believed to lower activation energies fordiffusion and promote steep concentration gradients (further increasingdiffusivity). Microwave sintering or microwave-assisted sintering areconsolidation techniques, typically at ambient pressure, which enhancesdensification because of rapid heating and homogenization of the part=sinternal temperature and creation of plasma at all powder asperities tocreate an enhanced surface.

Laser heating is an approach that can be used to primarily sinter a thinsection of powder (wherein the depth of penetration is very limited)and, hence, is often used for rapid prototyping machines that buildlayer by layer.

Electrical discharge heating is used to heat a component (within a hotpress) via electrical resistance. Typically, a hot press is employedsince constant contact (pressure) is needed and graphite adds in theelectrical conduction/heating of a component. When the electric filed ispulsed, plasma is generated therefrom at the asperities. Likewise,plasma sintering is similar in that an electromotive field is generatedresulting in an enhanced diffusion. A secondary type of plasma sinteringis to induce an external plasma using RF heating of gaseous species topromote localized heating and concentration gradients. However, thissystem is not as advantageous as the system described below due to thelack of applied pressure.

Plasma-assisted sintering is a technique whereby plasma is generatedwithin the powder compact. This plasma enhances surface activateddiffusion, which promotes densification at lower sintering temperaturesand/or promotes shorter sintering times. The instantaneous electricpulses using high currents generate the plasma. Often theplasma-assisted sintering is operated effectively applied to hotpressing, where the electric field pulses are deliver to the compactaxially through the use of graphite compaction rods. This technique isalso referred to as field-assisted sintering. Field strengths vary fordifferent materials, but generally range in from 18 to 50 V/cm.

A key feature of each of the above-described forming and sinteringmethods is that they enable formation of functionally-engineered wearsurfaces onto substrate interface surfaces that have nonplanar orotherwise irregular geometries. This feature is highly desirable, forexample, when the substrate is a portion of a drill bit, e.g., a cuttinginsert, characterized by having a curved or nonplanar interface surface.Additionally, green parts sintered according to the methods presentedavoid the undesired phenomena of material constituent migration, therebyensuring that the desired material compositions in the surface layer andsubstrate remain unchanged and that the desired related physicalproperties be retained.

1. A subterranean drill bit comprising a plurality of cutting elementsprojecting outwardly therefrom, at least one of the cutting elementsincluding a functionally-engineered cermet material disposed over asubstrate surface to provide a wear resistant surface thereon and havinga hardness that is different than that of the underlying substrate, thewear resistant surface being formed by the process of: forming aconformable cermet precursor material mixture comprising a polymerbinder; applying the conformable material mixture onto the surface ofthe substrate, wherein the substrate during the step of applying is in afully-densified state; and sintering the applied material mixture toform the wear resistant surface; wherein the sintered wear resistantsurface is a cermet having a material microstructure consisting of: afirst phase of grains selected from the group consisting of carbides,borides, nitrides and carbonitrides of W, Ti, Mo, Nb, V, Hf, Ta and Crrefractory metals; and a second phase of a binder material selected fromthe group consisting of Co, Ni, Fe and alloys thereof.
 2. The bit asrecited in claim 1 wherein before the step of applying, the conformablematerial mixture is preformed into a green-state part that is shaped tocomplement the surface the substrate, and the green-state part is placedover the substrate surface.
 3. The bit as recited in claim 1 wherein theconformable material mixture is applied onto the substrate in the formof a coating.
 4. A cutting element used for drilling subterraneanformations, wherein the cutting element comprises a substrate formedfrom a cermet material selected from the group consisting of refractorymetal carbides, nitrides, borides, carbonitrides and mixtures thereof,and the cutting element includes a functionally-engineered materialdisposed onto a surface of the substrate to form a wear resistantsurface having a hardness that is different than that of the underlyingsubstrate, the wear resistant surface being formed by the process of:applying a conformable material mixture onto an underlying substratesurface, wherein during such applying step the material mixture readilyconforms to the substrate surface and the substrate is in a preexistingfully-densified state; and sintering the material mixture to form thewear resistant surface wherein the sintered wear resistant surface is acermet having a material microstructure consisting of: a first phase ofgrains selected from the group consisting of carbides, borides, nitridesand carbonitrides of W, Ti, Mo, Nb, V, Hf, Ta and Cr refractory metals;and a second phase of a binder material selected from the groupconsisting of Co, Ni, Fe and alloys thereof.
 5. The cutting element asrecited in claim 4 wherein the substrate is formed from WC—Co.
 6. Thecutting element as recited in claim 4 wherein the step of sinteringoccurs at high pressure/high temperature conditions.
 7. The cuttingelement as recited in claim 4 wherein during the step of applying, theconformable mixture is provided in the form of a slurry.
 8. The cuttingelement as recited in claim 4 wherein the substrate surface that theconformable mixture is applied to is nonplanar.